The use of microwaves to dielectrically heat materials in a resonant cavity is well known in the prior art. In such systems, an energy source, located near the resonant cavity, generates microwaves which are in turn transmitted to the cavity through an associated waveguide. The material to be heated is typically placed adjacent to or in the resonant cavity and is directly heated by the applied microwave energy.
Several different types of resonant cavities have been used in the prior art depending on the heating application. One major type is a so called "multimode" cavity. This type of cavity generally has a square or rectangular cross-section with dimensions which are usually large compared to the microwave wavelength. The cavity is designed such that a large number of resonant standing wave modes propagate simultaneously, thus producing a relatively uniform electric field distribution throughout the cavity. A mechanical mode stirrer is often used in conjunction with a multimode cavity to further homogenize the electric field distribution. Because the dielectric heating effect is proportional to the square of the electric field strength, a relatively uniform heating effect is provided throughout the cavity.
Another type of cavity which has been used in microwave dielectric heating is a "single mode" cavity. This type of cavity is designed such that superposition of certain in-phase incident and reflected waves gives rise to a standing wave pattern which, for some simple structures, is well defined in the cavity. The precise knowledge of the electromagnetic field enables the user to place the dielectric material under treatment in a position of enhanced electric field strength for optimum transfer of electromagnetic energy. In general, for the same power applied, a single-mode resonant heating cavity establishes much higher electric field strengths than a corresponding multimode cavity. Single mode cavities are thus more advantageous than multimode cavities for heating low loss tangent dielectric materials. They are also, in general, very compact and have extremely high power densities.
While the use of single-mode and multimode resonant cavity devices for heating has been practiced for some time, the state-of-the-art in microwave heating is presently such that it has not been possible to use such devices to apply a plastic seal to the mouth of a container. Instead, prior art techniques for sealing a plastic sheet to a container mouth typically involve a process wherein a metallic/non-metallic laminate liner is first formed and inserted into a lip of a container closure. The closure is then attached to the container whereupon a low frequency electrical induction system is used to heat the metallic portion of the liner supported in the container closure. This operation causes the adjacent non-metallic portion of the liner to melt and thus seal the container mouth. This process, shown in U.S. Pat. No. 3,928,109 to Pollack et al, is disadvantageous because the metallic/non-metallic liner is costly to produce and because induction heating does not produce a satisfactory seal in a timely and economical manner.
It would therefore be advantageous to develop an apparatus for sealing a container which obviates these and other problems associated with prior art techniques and which enables a plastic sheet to be sealed directly to the container through application of electromagnetic energy.